1To make fish stock:
In a large saucepan, combine tuna and water. Bring to a boil. Add seaweed; return to boil. Simmer 15 minutes. Strain stock; discard seaweed. Fish may be used in a salad or other dish.
2Thaw shrimp, if using frozen; peel and remove vein.
In a small bowl, soak mushrooms in hot water to cover for 30 minutes; drain.
3In a large saucepan, combine 2 cups fish stock, shrimp, drained mushrooms, soy sauce, water chestnuts, parsley, and lime or lemon. Bring to a boil. Remove from heat; keep hot 5 minutes.